In general, rolled annealed copper foil has been used extensively in the flexible printed circuits industry. The grain structure and smooth surface is ideal for dynamic, flexible circuitry applications. However, rolled copper typically includes a horizontal grain structure, which can be more challenging for the etching of tight conductor spaces. In contrast, electrodeposited copper foil has a vertical grain structure that can be advantageous for obtaining tight etched spacing and well-defined conductor walls. The standard electrodeposited copper foil typically has a relatively high profile or rough surface as compared to rolled annealed copper foil, which can benefit bonding strength.
A typical device for manufacturing an electrodeposited copper foil comprises a metal cathode drum and an insoluble metal anode, the metal cathode drum being rotatable and having a mirror polished surface. The insoluble metal anode is arranged at approximately the lower half of the metal cathode drum and surrounds the metal cathode drum. A copper foil is continuously manufactured with the device by flowing a copper electrolytic solution between the cathode drum and the anode, applying direct current between these to allow copper to be electrodeposited on the cathode drum, and detaching an electrodeposited copper foil from the cathode drum when a predetermined thickness is obtained.
Copper foil manufactured in this manner is often used as a conductive material for printed wiring boards, including flexible printed circuits. Flexible printed circuits (FPC) refer to printed circuits in which the electronic components for the FPC are mounted or formed on a flexible substrate. As a result, the FPC can conform to a desired shape, or to flex during its use. FPCs have been used generally, for example, as wirings for bending portions of foldable (clamshell type) cellular phones, movable portions of digital cameras, printer heads, etc., and movable portions of equipment relevant to disks such as HDDs (Hard Disk Drives), DVDs (Digital Versatile Disks) and CDs (Compact Disks).
Therefore, at least where FPCs are involved, the flexibility of the copper foil is important from both reliability and manufacturing viewpoints. If the flexibility of the copper foil is not large enough, the bent or deformed copper foil will act as a spring and exert a restorative force against the flexible substrate of the FPC. This is referred to as the repulsive force of the copper foil. If the repulsive force is sufficiently high, the copper foil could delaminate from the flexible substrate during manufacturing or use. Further, when the FPC is connected to another device, a high repulsive force exerted by the copper foil against the flexible substrate could interfere proper bonding of the FPC to another component. Worse, the FPC could debond from this other component. Accordingly, solving the reliability problems and manufacturing problems in FPCs due to the flexibility of the copper foil are of particular interest in the copper foil industry.